Urinary System. The Kidney. Nephrons. Medical Physiology Z.H.Al-Zubaydi

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1 Lec.10 Medical Physiology Z.H.Al-Zubaydi Urinary System The kidneys alone perform the function and manufacture urine in the process. The other organs of the urinary system; the paired ureters and the single urinary bladder and urethra, provide temporary storage for urine or serve as transportation channels to carry it from one body region to another. The Kidney The kidneys are perfect example of hemostatic organs, which maintain the purity and constancy of internal fluids. They also regulate blood volume, blood pressure and stimulate RBCs production in bone marrow. Nephrons Each kidney contains over a million tiny structures called nephrons. Nephrons are the structural and functional units of the kidneys and responsible for forming urine. Each nephron consists of two main structures: a glonmerulus, which is a knot of capillaries, and a renal tubule. The closed end of the renal tubule is enlarged and cup-shaped and completely surrounds the glonmerulus. This portion of the renal tubule is called the glomerular or Bowman's, capsule. The inner (visceral) layer of the capsule is made up of highly modified octopus-like cells called podocytes. The rest of the tubule is about 3 cm long. As it extends from the glomerular capsule, it coils and twists before forming a hairpin loop and then again becomes coiled and twisted before entering a collecting tubule called the collecting duct. These different regions of the tubule have specific names; in order from the glomerular capsule they are the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT). Most nephrons are called cortical nephrons because they are located almost entirely within the cortex. In a few cases, the nephrons are called juxtamedullary nephrons because they are situated close to the cortex-medulla junction, and their loops of Henle dip deep into the medulla. The collecting ducts, each of which receives urine from many nephrons, run downward through the medullary pyramids, giving them their striped appearance. They deliver the final urine product into the calyces and renal pelvis. The glomerulus is both fed and drained by arterioles. The afferent 1

2 arteriole, which arises from an interlobular artery, is the "feeder vessel," and the efferent arteriole receives blood that has passed through the glomerulus. The glomerulus, specialized for filtration, is unlike any other capillary bed in the entire body. Because it is both fed and drained by arterioles, which are high-resistance vessels, and the afferent arteriole has a larger diameter than the efferent, blood pressure in the glomerular capillaries is much higher than in other capillary beds. This extremely high pressure forces fluid and solutes (smaller than proteins) out of the blood into the glomerular capsule. Most of this filtrate (99 percent) is eventually reclaimed by the renal tubule cells and returned to the blood in the peritubular capillary beds. The second capillary bed, the peritubular capillaries, arises from the efferent arteriole that drains the glomerulus. Unlike the highpressure glomerulus, these capillaries are low-pressure, porous vessels that are adapted for absorption instead of filtration. They cling closely to the whole length of the renal tubule, where they are in an ideal position to receive solutes and water from the tubule cells as these substances are reabsorbed from the filtrate percolating through the tubule. The peritubular capillaries drain into interlobular veins leaving the cortex. Urine Formation Urine formation is a result of three processes; filtration, tubular reabsorption, and tubular secretion. Each of these processes is illustrated in Figure 1. Filtration is nonselective, passive process. The filtrate that is formed is essentially blood plasma without blood proteins. Both proteins and blood cells are normally too large to pass through the filtration membrane (when either of these appear in the urine, there is some problem with the glomerular filters). As long as the systemic blood pressure is normal, filtrate will be formed. If arterial blood pressure drops too low, the glomerular pressure becomes inadequate to force substances out of the blood into the tubules, and filtrate formation stops. Tubular Reabsorption Besides wastes and excess ions that must be removed from the blood, the filtrate contains many useful 2

3 substances (including water, glucose, amino acids, and ions), which must be reclaimed from the filtrate and returned to the blood. Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule. The tubule cells are "transporters" taking up needed substances from the filtrate and then passing them out their posterior aspect into the extracellular space, from which they are absorbed into peritubular capillary blood. Some re-absorption is done passively (for example, water passes by osmosis), but the reabsorption of most substances depends on active transport processes, which use membrane carriers and are very selective. Needed substances (for example, glucose and amino acids) are usually entirely removed from the filtrate. Nitrogenous waste products are poorly reabsorbed. These include urea, formed by the liver as an end product of protein breakdown when amino acids are used to produce energy; uric acid, released when nucleic acids are metabolized; and creatinine, associated with creatine metabolism in muscle tissue. Various ions are reabsorbed or allowed to go out in the urine, according to what is needed at a particular time to maintain the proper ph and electrolyte composition of the blood. Tubular Secretion is essentially reabsorption in reverse. Some substances, such as hydrogen and potassium ions and creatinine, also move from the blood of the peritubular capillaries through the tubule cells or from the tubule cells themselves into the filtrate to be eliminated in urine. This process seems to be important for getting rid of substances not already in the filtrate, such as certain drugs, or as an additional means for controlling blood ph. 3

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5 Fig. 1 : Scheme of single large uncoiled nephron. [ a: Filtration, b: Tubular Reabsorption and c: Tubular Secretion] Characteristics of Urine In 24 hours, the marvelously complex kidneys filter some 150 to 180 liters of blood plasma through their glomeruli into the tubules, only about 1.0 to 1.8 liters of urine are produced. Filtrate contains everything that blood plasma does (except proteins), but by the time it reaches the collecting ducts, the filtrate has lost most of its water and just about all of its nutrients and necessary ions. What remains, urine, contains nitrogenous wastes and unneeded substances. Freshly voided urine is generally clear and pale to deep yellow. The normal yellow color is due to urochrome, a pigment that results from the body's destruction of hemoglobin. Urine ph is usually slightly acid (ph 6), but changes in body metabolism and certain food may cause it becomes more acidic or basic. Solutes normally found in urine include sodium and potassium ions, urea, uric acid, creatinine, ammonia, bicarbonate ions, and various other ions, depending on blood composition. With certain 5

6 diseases, urine composition can change dramatically, and the presence of abnormal substances in urine is often helpful in diagnosing the problem. This is why a routine urinalysis should always be part of any good physical examination. Substances not normally found in urine are glucose, blood proteins, red blood cells, hemoglobin, white blood cells (pus), and bile. The ureters The ureters are slender tubes running from each kidney to the bladder. They conduct urine by peristalsis from kidney to bladder. The bladder The bladder is a muscular sac posterior to the pubic symphysis. It has two inlets (ureters) and one outlet (urethra). In males, the prostate gland surrounds its outlet. The function of the bladder is to store urine. The urethra The urethra is a tube that leads urine from the bladder to the body exterior. In females, it is 3 to 4 cm long and conducts only urine. In males, it is 20 cm long and conducts both urine and sperm. The internal sphincter of smooth muscle is at the bladder-urethra junction. The external sphincter of skeletal muscle is located more inferiorly. Micturition is emptying of the bladder. The micturition reflex causes the involuntary internal sphincter to open when stretch receptors in the bladder wall are stimulated. Since the external sphincter is voluntarily controlled, micturition can ordinarily be temporarily delayed. Incontinence is the inability to control micturition. 6